Japanese Laid-Open Patent Publication No. 4-116234 discloses, as one type of internal combustion engine, a flexible fuel internal combustion engine that is capable of using fuel containing alcohol, which is prepared by blending alcohol with gasoline at a given ratio. The content of carbon atoms in the alcohol is different from the content of carbon atoms in normal fuel such as gasoline. It is thus necessary for the flexible fuel engine to regulate the fuel injection amount depending on which type of alcohol is blended with the gasoline and in correspondence with the concentration of the alcohol. The stoichiometric air-fuel ratio of a blended fuel containing ethanol is smaller than the stoichiometric air-fuel ratio of the gasoline. Thus, if the blended fuel containing ethanol is used, the injection amount of the blended fuel must be increased to such a value that the oxygen concentration of the exhaust gas generated from the blended fuel becomes equal to the oxygen concentration of the exhaust gas produced through combustion of the gasoline by the stoichiometric air-fuel ratio. Specifically, by correcting the fuel injection amount in correspondence with the alcohol concentration, sufficient purification performance of a catalyst device provided in an exhaust passage is ensured and deterioration of the properties of the exhaust gas is suppressed. To perform such correction of the fuel injection amount, the flexible fuel engine learns the alcohol concentration of the blended fuel using a detection value of an oxygen concentration sensor arranged in the exhaust passage. The engine operates to correct the fuel injection amount based on a learned value.
However, the alcohol concentration of the fuel combusted by the flexible fuel engine may be inconstant. In other words, the engine may receive a blended fuel with an alcohol concentration different from that of the fuel currently retained in the fuel tank. In other words, in operation of the engine after refueling, the alcohol concentration of the blended fuel significantly changes. Nonetheless, once a learning procedure of the alcohol concentration is completed, the fuel injection amount may be corrected in correspondence with the current alcohol concentration of the fuel even after the alcohol concentration of the blended fuel retained in the fuel tank has changed. However, the learning procedure of the alcohol concentration cannot be resumed unless a prescribed condition is satisfied so that the accuracy of the learning procedure is ensured. Further, even if the condition is met and the learning procedure is started, a certain amount of time is necessary before the procedure is completed. That is, if the alcohol concentration of the blended fuel changes due to the refueling, the engine cannot perform correction of the fuel injection amount in correspondence with the alcohol concentration until after the predetermined time elapses. This deteriorates the air-fuel ratio.